1. Field of the Invention
The present invention relates to a data communication system using a session key in the transaction executed between, for example, a host device and a plurality of IC cards.
2. Description of the Related Art
Conventionally, in a data communication system including a host device, a terminal, and the like, data was encrypted using predetermined key data (session key data) and the encrypted data was transmitted in order to enhance the security of the data. To make the security much higher, a method of encrypting the session key data and transmitting the encrypted session key data, has recently been proposed. A preceding session key, which has been used once, can be practically used again as an encrypting/decrypting key for transmission of current session key data. No problems will occur in this case if two apparatuses for sending and receiving the data are fixed or predetermined. In the transaction executed between one host device and a plurality of IC cards, however, the final session keys for the respective IC cards must be all stored on the host device side for succeeding transactions.
A memory device for storing all the final session keys is needed and therefore a burden of the host device is increased.